Computer modelling of block copolymers under external fields

Dessi, Roberta (2016) Computer modelling of block copolymers under external fields. PhD thesis, University of Lincoln.

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Item Type:Thesis (PhD)
Item Status:Live Archive


This thesis reports on modelling the morphological of Block Copolymers (BCPs) by
using computer simulation. In this work, the Cell Dynamics Simulation (CDS) method
is used, which is a good compromise between computational speed and physical accuracy.
First, it has been performed a 3-dimensional study of diblock copolymer standing up
cylinders in two different types of confinement: topographical and chemical patterns.
In the case of square walls of small sizes the system is dominated by the walls, inducing
a 2x2 square lattice of cylinders, while in large boxes the system is dominated by
hexagonal packing. For intermediate sizes topographical confinement has a stronger
influence than chemical pattern confinement and can induce a better system of 3x3
square lattice cylinders. For square confinement compatible with a 4x4 square lattice
the tetragonal phase is observed to be a transient system leading towards a twisted
hexagonal packing. Simulations have also been performed in a diamond lattice which
can naturally accommodate hexagonal packing, and rectangular boxes which can induce
better orientation of the hexagonal lattice along the direction parallel to the long
Next, diblock copolymer cylinder-forming thin films confined between two parallel selective
homogeneous walls have been investigated. By changing the value of the surface
field and the value of the film thickness several morphologies have been observed. In
order to tailor desired structures some of these morphologies are studied under a simple
steady shear flow. Shear flow is observed to induce a better hexagonal packing of a
monolayer of perforated lamellae and of a monolayer of cylinders perpendicular to the
thin film plane. A monolayer of cylinders parallel to the thin film plane with random
orientation is found to align perfectly in the shear flow direction. Further increase of
the shear rate induces a phase transition: from one perforated lamellae layer to one
lamellae layer; from a double layer of perforated lamellae to parallel cylinder layers; and
from a monolayer of cylinders perpendicular to the thin film plane to two half parallel
cylinder layers.
In the end, self-assembly of lamellae-, cylinder-, bicontinuous, and sphere-forming diblock
copolymers in spherical confinement is studied. The effects of different confine-
ment size and selective surface are examined systematically. The simulations reveal that
a rich variety of morphologies, ranging from onion-like structures, perforated lamellae,
helices structures, and the coexistence of perforated and lamellae structures, can be
formed spontaneously from a randomly generated initial state. The structure diagrams
of lamellae-forming diblock copolymers show that the morphologies obtained with a
selective surface are similar to those obtained with a negative selective surface, but
different to those found with a neutral selective surface. The structural diagrams of
bicontinuous-, sphere-, and cylinder-forming diblock copolymers show that the morphologies
found for surfaces selective for different blocks are different from each other.
In the case of bicontinuous-forming diblock copolymers the morphological behaviour
obtained with a neutral surface is similar to those obtained when the surface is selective
for the minority block. In the case of sphere-forming diblock copolymers the
morphologies obtained with a neutral surface are similar to those found with surface
selective for the majority block. Instead, for cylinder-forming diblock copolymers the
morphology obtained under a neutral surface is totally different from those obtained
when the surface is selective.

Keywords:Diblock co-polymer
Divisions:College of Science > School of Mathematics and Physics
ID Code:23704
Deposited On:24 Aug 2016 12:43

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